Method and solution for applying a phosphate conversion coating on ferriferous surfaces



United States Patent 3,129,123 METHOD AND SGLUTION FOR APPLYING APHOSPHATE CONVERION COATING 0N FER- RH ERGUS SURFACES Edward A.Rodzewich, Flour-town, Pa., assignor t9 Arnchern Products, Inc., Ambler,Pa, a corporation of Delaware No Drawing. Filed Apr. 4, 1962, Ser. No.184,961

7 Claims. (Cl. 148-615) The present invention relates to the art ofproducing a phosphate conversion coating on a ferriferous metal surfaceand is particularly concerned with that type of conversion coating whichis produced by treating the surface of the metal with alkali metalphosphate solutions.

In this disclosure the term alkali metal phosphate is to be understoodas including both sodium and potassium phosphates as well as ammoniummono and ammonium dihydrogen phosphates.

In this art as heretofore practiced the alkali metal phosphate coatingbaths have been employed as aqueous solutions having pH values rangingfrom approximately 3.2 to 6.2 although, as a general rule, a somewhatnarrower pH range has been employed, say from about 4.7 to 6.0.Furthermore, it has been customary practice to include one or more ofthe so-called accelerating agents such, for example, as chlorates,nitrates, nitrites and peroxygen compounds which, as is well known,operate to reduce the time required for coating formation.

In prior practice another factor has also been important and that isthat the type of solutions just described have had to be heated totemperatures ranging from about 170 to 180 F. in order that they mayproduce the desired coatings within a reasonable period of timesay from1 to 3 minutes.

Other problems have also arisen in connection with the practices of theprior art. For instance, while nitrite ion has been recognized as anaccelerating agent in baths of the type described, its use, generallyspeaking, has been restricted to combinations with other well knownaccelerators. This restricted use of the nitrite ion is a reflection ofcertain inherent disadvantages associated with its behavior in coatingsolutions of this type. For example, where an insufficient amount ofaccelerator is employed, no apparent coating is produced on ferriferoussurfaces. Conversely, if too great an amount is employed a phenomenonknown in the art as white spot is produced on the metal being treatedand, eventually, no coating of any appreciable value can be obtained.

With nitrite accelerated alkali metal phosphate coating solutions it hasbeen observed also that the coatings produced are lighter in weight byas much as 20% to 25% as compared to coatings resulting from the use ofchlorates or peroxygen compounds as accelerators, which lighter weightcoatings are not as suitable for pre-paint use since their corrosionresistance is definitely inferior to that of heavier coatings.

More recently it has been found that the use of bromates as acceleratingagents not only yields heavier coating weights than are obtained withnitrites but also permits the use of lower reaction temperatures on theorder of 150 to 160 F., although some sacrifice in coating weightresults from the use of such lower bath temperatures. However, bromateaccelerated baths often produce coatings which are mottled or otherwiseuneven in appearance, which uneven appearance frequently affects theappearance of the treated metal surface after it has been given a finalsiccative finish such as paint or the like.

With all of the foregoing limitations of the prior art in mind theprincipal object of the present invention may be said to reside in theprovision of an improved nitrite accelerated alkali metal phosphatecoating process by means of which it is possible to produce, onferriferous metal surfaces, highly corrosion resistant and extremelyeffective paint-bonding coatings at lower temperatures than has beenpossible heretofore.

A concommitant object of this invention is the provision of a solutionfor and a method of applying phosphate conversion coatings toferriferous metal surfaces of more uniform appearance and of greaterweight than has ever been possible heretofore with any of the nitriteaccelerated alkali metal phosphate coating baths familiar to the art.

How the foregoing objects and advantages as well as others which mayappear hereinafter are attained will become apparent in connection withthe following description.

The present invention is based upon the discovery that if at least 0.05gram/liter of a hydrazine compound having the formula:

H H Adlai-B wherein A is selected from the group consisting of hdrogen,the phenyl radical and unsubstituted alkyl radicals of from 1 to 3 totalcarbon atoms, and B is selected from the group consisting of hydrogen,the

radical and the fi-NH radical, is added to a nitrite accelerated alkalimetal phosphate coating solution having a pH of from 4.0 to 5.8, suchsolution can be employed as a coating bath in the treatment offerriferous metal surfaces to produce an extremely uniform, highlycorrosion resistant coating of substantially greater Weight thanheretofore possible in this art even when customary or normal treatingcycles of 1 to 3 minutes are employed and, furthermore, that theseimprovements in result can be obtained at appreciably lower coating bathtemperatures than have ever been practical heretofore.

As noted hereinabove, at least 0.05 gram/liter of the specifiedhydrazine compound must be incorporated into the coating solutions ofthis invention. Where less than this minimum amount is utilized noappreciable coating will result and the benefits of this invention willbe completely lost.

Although use of as little hydrazine compound as 0.05 gram/liter yieldsthe desired coatings at lower operating temperatures, it has beendiscovered that 0.1 gram/ liter is a preferred minimum in order toinsure, at all times throughout operation of the bath, that a sufficientamount of this additive is present.

So far as the upper limit of hydrazine compound is concerned it has beenfound that there is no apparent deleterious effect from the use of asmuch as 25 grams/ liter of these compounds. Use of even higher amountsthereof are limited in some instances by solubility considerations, butwhere a particular hydrazine falling within the generic formula listedhereinabove is soluble in an infinite degree, such, for example, asmethyl hydrazine, no limitation on the amount used has been found. Inthe interests of economy, however, it is preferred not toexceed about 10grams/liter of hydrazine compound in the coating solutions of thisinvention.

Typical hydrazines which fall within the generic structure listed aboveinclude hydrazine, methyl, ethyl and propyl hydrazines, phenylhydrazine, semicarbazide, betaaceto-phenylhydrazine andbeta-aceto-methylhydrazine.

Molecular weight of hydrazine compound used X46 B. For the maximumamount of N Molecular weight of hydrazine compound used Where thequantity of hydrazine compound employed is greater than approximately 1gram/liter it is necessary to employ not less than the minimum quantityof nitrite ion as calculated by Equation A, While at the same time themaximum quantity of N0 may go well beyond the amount which is determinedby Equation B. Indeed, just as soon as more than approximately 1gram/liter of hydrazine compound is employed there seems to be no needto limit the quantity of nitrite ion which can be utilized in the bathand as much as grams/liter of N0 has been found to produce completelysatisfactory results. The only limitation on the upper quantity wouldseem to be imposed by considerations of economy and unnecessary wastageof this salt. Furthermore, I have found that use of relatively largeexcesses of nitrite ion relative to the amount of hydrazine compoundemployed where the content of the latter is approximately 1 gram/literor more has, somewhat surprisingly, made it possible to greatly increasecoating weights without impairment of their quality. In short, with myprocess I find that coating weights can be substantially increased byincreasing the quantity of nitrite employed although this advantagecannot be realized to its fullest extent except Where the bath containsmore than approximately 1 gram/liter of the hydrazine compound.

To refer again to Equations A and B, in situations where the quantity ofhydrazine compound lies between 0.05 and 1 gram/liter the followingspecific example is suggested. Where phenyl hydrazine is employed and isutilized in an amount of from 0.05 to 1 gram/liter, then the amount ofnitrite ion which is required must lie between the limits determined bysubstituting the molecular weight of phenyl hydrazine (108.14) in theequations. This will give a range for the nitrite (N0 of 0.02 to 0.42gram/ liter of coating solution. However, where the amount of hydrazinecompound employed is greater than 1 gram/liter it is only necessary tobe sure that at least 0.02 gram of N0 per liter is employed, althoughmany times more than the maximum of 0.42 gram/liter is entirelypractical and, as stated above, as much as 10 grams/ liter of N0 havebeen found to produce completely satisfactory results. This fact wascompletely unpredictable insofar as prior art experience with nitriteaccelerated alkali metal phosphate coating solutions is concerned, and Ihave discovered that by increasing the nitrite content coating weightsof more than twice that normally realized heretofore can be obtainedwith my invention.

The nitrite ion may be introduced into the alkali metal phosphatecoating solution as a salt, such, for example, as an alkali or alkalineearth metal salt. Due to commercial availability the sodium salt ispreferred. However, the only limitation on the salt used is that thecation portion thereof exhibit no deleterious effect upon the coatingreaction.

So far as control over the nitrite content of the improved phosphatecoating solutions of this invention is concerned, this is readilyaccomplished either by a permanganate titration in acidic medium or byan iodometric titration according to well established art practices.

With respect to control of the hydrazine compound content of my improvedcoating solutions, it has been found that a visual control is bothadequate and reliable so long as the coating solution contains nitriteion and is not deficient in alkali metal phosphate ion or pH. Forexample, where coating quality begins to deteriorate as evidenced bythinner coatings being produced on ferriferous surfaces, 21 nitritetitration is immediately taken and, if such titration shows the presenceof nitrite ion, it is only necessary to add at least 0.05 gram/liter ofa hydrazine compound in order to restore the bath to its original anddesired coating ability. Where the nitrite titration shows a deficiencyof this accelerator, it is preferred practice to add nitrite withoutadding hydrazine compound, since results with this process show that thehydrazine compounds are not consumed in the coating reaction, and thatthe only loss thereof is through entrainment or drag-out during coatingoperations.

As noted hereinabove, the alkali metal phosphate coating solutions ofthis invention must be maintained within a pH range of 4.0 to 5.8. WherepH values of less than 4.0 are employed the solution exhibits anundesirable etching action on the ferriferous metal surfaces therebyimpairing coating formation. Conversely, where the solution pH ispermitted to rise above about 5.8, the coatings produced will be foundto be thin and powdery, while still further pH increases will result inno coating being produced upon the metal surfaces. A preferred pH rangefor operating the process of this invention has been found to be from4.5 to 5.7 since optimum coatings are produced within this narrower pHrange.

Adjustment of the coating solution pH may be made, where needed, by theaddition of small increments of either phosphoric acid or sodiumhydroxide according to well established art practices.

One of the outstanding improvements derived from the process of thisinvention is the ability to obtain coatings on ferriferous metalsurfaces at temperatures of from to F. As noted hereinabove prior usageof alkali metal phosphate coating solutions necessitated the employmentof coating temperatures of to F. in order to obtain satisfactoryresults. The reduced operating temperatures made possible by theimprovements of this invention represent considerable economicadvantages to the coating industry.

The 120 to 150 F. operating temperature range applies to dip, spray orroller coating applications and, while no harmful results are obtainedby use of coating temperatures above 150 F., that is temperatures of 170to 180 F., or even higher, excessive temperatures have been found toresult in reduced coating weights, and such temperatures are completelyunnecessary and represent an economic waste when utilizing the presentprocess. Temperatures below the minimum of 120 F. should not be usedsince it has been found that the desired coating weights will not beobtained at such low temperatures within reasonable operating cycles.

So far as the length of time of treatment is concerned, it has beenfound that the coating cycle will provide the desired coatings whenoperated for as little time as 30 seconds to as much time as 5 minutes.However, the preferred operating cycle is from 1 to 3 minutes utilizinga temperature of from about 135 to 145 F. Use of lower treatingtemperatures will necessitate longer contact times, while, conversely,use of slightly higher treating temperatures will require shortercontact cycles.

In order to contrast the results obtainable with the process of thepresent invention with results obtained from a typical prior art nitriteaccelerated alkali metal phosphate coating solution, the following testswere conducted:

A tap water solution was prepared containing, per liter, 10 grams ofmonosodium phosphate (NaH PO The pH of this solution was 5.2.

Portions of this alkali phosphate solution were then utilized for thetreatment of clean, cold-rolled steel panels utilizing contact cycles of2 minutes and coating temperatures of 140 F., except for Example #2which utilized a temperature of 175 R, which latter temperature istypical of that used in the prior art. Hydrazine compounds and nitriteaccelerator (as NaNO were added to these solutions prior to coatingutilization in accordance with the following table:

Table I Hydrazine Compound Grams N03 C.W., Panel Example per Mg./Appearance Type Grams/ liter it.

liter streaked, non- }uniform redgold coating.

phenylhydrazine. 0.2 0.085 55 Uniform blue coating. phenylhydrazine. 0.40.17 61 Uniform deep blue coating. phenylhydrazine- 0. 9 0. 38 65 Do.methylhydra- 0. 65 0. 65 50 Do.

zine. hydrazine 0. 45 0. 65 54 Do. phenylhydrazine. 0.2 1.5 Uniformbluish-green coating.

Table II Corrosion Results tcr rs. (ASTM-B 117571) Average Rating Panelsfrom Example From the foregoing tests it is readily apparent that theuse of nitrite accelerated alkali metal phosphate coating solutions,which solutions also contain an amount of hydrazine as specified,produced coatings of higher weight and superior corrosion resistance atlower temperatures than could heretofore be obtained with nitriteaccelerators.

It is within the purview of this invention to employ an amount ofwetting agent in the improved alkali metal phosphate coating solutionsof this invention, if desired, since, as is well known in the art, suchwetting agents serve to improve the solutions ability to wet thoroughlythe entire metal surfaces introduced therein.

1 claim:

1. The method of forming a phospate conversion coating on a fenriferoussurface which comprises the following steps:

(A) Applying to the surface an aqueous solution consisting essentiallyof:

(1) alkali metal phosphate from the class which consists of sodium,potassium, and ammonium mono and dihydrogen phosphates in coatingproducing concentration,

(2) from 0.05 to approximately 1 gram/liter of hydrazine compound havingthe formula:

AN-NB where A is selected from the group consisting of hydrogen, thephenyl radical and unsub- 6 stituted alkyl radicals of from 1 to 3 totalcarbon atoms, and B is selected trom the group consisting of hydrogen,the

-CCH

radical and the lfNH,

radical and (3) nitrite ion (calculated as N0 Within the rangedetermined by the following equations:

For the minimum amount of N0 0.05 Molecular weight of hydrazine compoundused For the maximum amount of N0 1 Molecular weight of hydrazinecompound used (B) maintaining the pH of said solution at from 4.0

to 5.8; and

(C) maintaining the temperature of the solution at not less than F.

2. The method of claim 1 wherein the temperature of the solution ismaintained at from 120 F. to 150 F.

3. The method of claim 1 wherein the temperature of the solution ismaintained at from F. to F. and the treatment continued for from 1 to 3minutes.

4. The method of claim 1 wherein the pH is maintained from 4.5 to 5.7.

5. The method of forming a phosphate conversion coating on a ferriferoussurface which comprises the following steps:

(A) Applying to the surface an aqueous solution consisting essentiallyof (1) alkali metal phosphate from the class which consists of sodium,potassium, and ammonium mono and dihydrogen phosphates in coatingproducing concentration, (2) upwards of 1 gram/liter of hydrazinecompound having the formula:

wherein A is selected from the group consisting of hydrogen, the phenylradical and unsubstituted alkyl radicals of from 1 to 3 total carbonatoms, and B is selected from the group consisting of hydrogen, the

CCH

radical and the radical and (3) a quantity of nitrite ion (calculated asN0 which is not less than 0.05 Molecular weight of hydrazine compoundused 7 monium mono and dihydrogen phosphates in coating producingconcentration and (2) the following ingredients as addition agents:

(A) not less than 0.02 gram/liter of nitrite ion (calculated as N and(B) not less than approximately 1 gram/liter of hydrazine compoundhaving the formula:

ANNB

wherein A is selected from the group consisting of hydrogen, the phenylradical and unsubstituted alkyl radicals of from 1 to 3 total carbonatoms, and B is selected from the group consisting of hydrogen, the

radical and the radical.

7. A bath for use in forming a phosphate conversion coating on aferriferous surface, said bath consisting essentially of:

(1) an aqueous alkali solution of phosphates from the class whichconsists of sodium, potassium, and arm monium mono and dihydrogenphosphates in coating producing concentration and (2) the followingingredients as addition agents:

radical and the f :-NH,

radical and (B) nitrite ion (calculated as N0 the quantity of ion beingwithin the range determined by the following equations:

For the minimum amount of N0 0.05 Molecular weight, of hydrazinecompound used For the maximum amount of N0 1 Molecular weight, ofhydrazine compound used References Cited in the file of this patentUNITED STATES PATENTS Logue et a1. June 24, 1958 UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No, 3 129,123 April 14 1964Edward A. Rodzewich error appears in the above numbered pat- It ishereby certified that id Letters Patent should read as ent requiringcorrection and that the sa corrected below.

line 67 strike out "not less than 120 F0" Column 6,

ad to 598; and

and insert inste Signed and sealed this 4th day of August 19640 (SEAL)Attes t:

EDWARD J. BRENNER ERNEST W SWIDER Attesting Officer Commissioner ofPatents

1. THE METHOD OF FORMING A PHOSPATE CONVERSION COATING ON A FERRIFEROUSSURFACE WHICH COMPRISES THE FOLLOWING STEPS: (A) APPLYING TO THE SURFACEAN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF; (1) ALKALI METALPHOSPHATE FROM THE CLASS WHICH CONSISTS OF SODIUM, POTASSIUM, ANDAMMONIUM MONO AND DIHYDROGEN PHOSPHATES IN COATING PRODUCINGCONCENTRATION, (2) FROM 0.05 TO APPROXIMATELY 1 GRAM/LITER OF HYDRAZINECOMPOUND HAVING THE FORMULA: